DX should work the same way as K3, but you need a DX firmware image instead of K3. The different serial connector location and pinout is shown in Dev corner / master index / DX index.

Before the "simple debricking" method was developed for K4 and K5, the serial port was the ONLY way to debrick or flash kindle firmware backups (even for the K4 and K5).

The video shows that two ports are used for the Kindle Keyboard to unbrick it (the USB port and the port that powers the light). Do you know if the Kindle DX have two ports too? If not, can everything be done using just the USB port?